Conventionally, as a metallic can that is subjected to severe forming, such as a thin-wall deep drawn can or a drawn and ironed can (DI can), a can provided with a resin layer on the internal surface thereof is generally used to prevent a decrease in taste and flavor and hence a change in property of the content due to dissolution of metal from a metal sheet. As such a can that is provided with a resin layer on the internal surface thereof and a resin-coated metal sheet capable of forming such a can, a metal sheet to which polyester resin is laminated is used as a substitute for epoxy coating layer of which danger of endocrine disrupter has recently been pointed out.
The polyester resin used in such an application is required to provide the following performance: The adhesion to a metal sheet is good, so that deformation such as elongation and compression of film caused by forming at the can manufacturing time and deterioration of film and decrease in adhesion due to friction do not occur; a polyester resin film is not crystallized or deteriorated after heat treatment such as drying, printing, and retorting, and hence separation and shrinkage of film, cracks, pinholes, etc. do not occur; a polyester film is not cracked or separated by a impact to a can; corrosion or peeling does not occur when polyester resin comes into contact with various kinds of contents; a film is not whitened; and so on. Furthermore, it is required that flavor components of content in a can be not adsorbed or absorbed to a polyester film, or the flavor of content be not impaired by the dissolved components of polyester film and smell (hereinafter referred to as flavor property).
For example, in Unexamined Japanese Patent Publication No. 59-232852, a metal sheet coated with polyethylene terephthalate resin is proposed from a viewpoint of formability, heat resistance, corrosion resistance, flavor property, etc. Also, in Unexamined Japanese Patent Publication No. 1-180336, a metal sheet coated with polybutylene terephthalate resin is proposed. Furthermore, excellent formability that withstands drawing and ironing is required, and also excellent adhesion such that a resin film does not separate from a steel sheet and impact resistance such as to withstand shocks at the can manufacturing time, the canning time, and transportation time are required. To meet the requirements, Unexamined Japanese Patent Publication Nos. 5-269920 and 6-320669 have disclosed a manufacturing method in which excellent formability, adhesion, and impact resistance can be obtained in addition to food sanitation property and flavor property, which a polyethylene terephthalate resin film inherently has, by controlling the crystalline orientation of film by means of laminating technology or the like. Such a technology can be applied to the present requirement level for formability, adhesion, and impact resistance.
However, in the concerned field, the decrease in sheet thickness is progressing year by year, and this tendency is thought to continue in the future as well. In the above-described manufacturing method for polyethylene terephthalate resin, it is difficult to achieve both formability and impact resistance when the resin film is subjected to severer forming. The reason for this is that formability and impact resistance depend greatly on the crystalline orientation (plane orientation), and have a mutually contradictory tendency. Specifically, if a crystalline orientation component increases in a resin layer, plastic deformation is hindered by a crystalline portion, and hence formability is deteriorated. Therefore, from the viewpoint of formability, the smaller amount of oriented crystals is, the better formability is. However, since this crystalline portion acts as a portion for stopping the progress of crack, from the viewpoint of impact resistance, the larger amount of oriented crystals is, the higher impact resistance is. Thus, formability and impact resistance are designed by regulating the amount of oriented crystals so that both properties are in the allowable region. However, the region in which both properties are compatible is the present limit of required performance, so that it has been expected that a new highly formable film that can respond to an increase in the degree of forming in the future, and a resin laminated metal sheet coated with this film will be developed.
In order to satisfy such a need, studies have been conducted earnestly on a technology in which both high formability and impact resistance are achieved by mixing polyolefin resin with polyester resin. Unexamined Japanese Patent Publication Nos. 7-195617 and 7-195618 have disclosed a technology in which a film consisting of a composition of saturated polyester resin and ionomer resin is laminated to a metal sheet. Although these publications indicate that impact resistance can be kept even in an amorphous state, the impact resistance obtained merely by addition of ionomer is insufficient, and inversely, the orientation of polyester resin is actually hindered by the addition of ionomer, which poses a problem in that the mechanical strength that polyester resin inherently has decreases, and resin is broken at the forming time. Also, the adhesion to a substrate metal after forming or after heating is also inferior.
Furthermore, Japanese Patent Publication Nos. 7-290643 and 7-290644 have disclosed that impact resistance can be improved by laminating a ternary composition of polyester resin, polyester elastomer, and ionomer resin to a metal sheet. However, the mixture of this composition also has low capability for relaxing impact stress of polyester elastomer, so that the effect of improving impact resistance of polyester resin is very little. Also, as in the case where elastomer is not added, the adhesion to a substrate metal after forming or after heating is also inferior.
On the other hand, WO 99/27026 Publication has disclosed a technology in which an elastomer resin encapsulated by a vinyl polymer having a polar group is finely dispersed in polyester resin. Such fine dispersion of elastomer resin improves impact resistance, so that the level of formability and impact resistance is high. However, the control of process for manufacturing such a capsule state is difficult to carry out, and the dispersed state of elastomer resin changes greatly depending on the resin forming conditions, and is unstable. Therefore, there arises a problem in that the performance of the resultantly obtained resin film is not always constant. It is found that if such a dispersed state deviates from the optimum conditions, the performance decreases greatly. Even if a resin of such a composition is actually manufactured, a portion where the resin performance is low is produced partially, so that the obtained performance such as formability and impact resistance is insufficient as a whole. Furthermore, there arise problems of decreased adhesion to a substrate metal due to forming or impact and decreased adhesion after heating.
Furthermore, Unexamined Japanese Patent Publication No. 2001-172481 has disclosed a technology in which the decomposition of vinyl polymer is restrained by adding an appropriate amount of an oxidation inhibitor (free radical inhibitor) to a mixed resin in which a vinyl polymer having a polar group is finely dispersed in polyester resin. However, in a particular mixture of vinyl polymer and polyester resin, when a polymerization catalyst of polyester resin and an oxidation inhibitor coexist, resin is liable to be deteriorated. In particular, polyester resin is deteriorated, and performance such as impact resistance and flavor property is decreased.
Still further, Unexamined Japanese Patent Publication No. 2001-353814 has disclosed a technology in which a resin layer in which fine ionomer resin exists in polyester resin as a dispersion phase is coated, by which the impact resistance and adhesion are enhanced. However, the compositions of dispersed modified polyolefin resin and polyester resin are improper, so that the performance is insufficient.
Also, a can using a metal sheet to which such a resin film is laminated has a problem in that the film is whitened after retorting and thereby the appearance is deteriorated. In particular, a can lid and a can bottom, which come into direct contact with retort steam, remarkably is whitened, and such a phenomenon cannot be prevented by means of the above-described prior art.